Water is an unavoidable contaminant in fuel. Water can affect components in fuel systems and lead to operational delays and increased maintenance activities. In addition, the propensity for microbiological contamination is directly proportional to the presence of water and the temperature within fuel tanks.
Although water may affect fuel systems of land or water based vehicles, water is a particular problem in aircraft fuel systems. Water may enter aircraft fuel tanks from fuel loaded into the aircraft fuel tanks during refuel (dissolved water) and from air entering the aircraft fuel tanks via its vent system. A vent system to ambient air is normally required to normalise the pressure within the fuel tanks during climb and descent of the aircraft.
Dissolved water poses little problem to fuel systems reliability and function so long as it remains dissolved in fuel. However, since the solubility of water in fuel decreases with decreasing temperature, during cruise water from dissolution (rejection of dissolved water from fuel) can form droplets of the order of microns suspended within the fuel. These water droplets settle slowly to the bottom of the tank. In addition, natural convection currents bring the saturated fuel into contact with cold tank surfaces where water from dissolution causes condensation on cold surfaces. The condensation tends to run down the walls of the fuel tank and collect as free water pools in the bottom of the tank.
Pooled free water can be drained off when the aircraft is on the ground but this is time consuming and costly, leading to a loss of operational efficiency. At sufficiently cold temperatures the free water can freeze, which can require a further significant maintenance operation of heating the aircraft in a hangar in order to perform the water drain activity. After safety, one of the most important criteria in aircraft design is the aircraft operational cost. Aircraft operational cost can be significantly impacted by its maintenance requirement and water contamination within fuel tanks is a maintenance burden, which this invention seeks to reduce.